Early childhood caries (ECC) is a common infectious disease among infants and toddlers. It was initially labeled "baby-bottle tooth decay" given its association with bottle-feeding. Based on two case reports in 1977 that breastfeeding may also be responsible for caries development, the name changed initially to "nursing caries" and later to ECC. Since these initial reports appeared, few studies have mechanistically addressed breast milk's role in caries development, and those that have found no association. Similarly, there is no epidemiological evidence for an association between breastfeeding and caries, which would be counterintuitive given breastfeeding's protection against infection, human milk's numerous antimicrobial components, and the negligible caries frequency among children in cultures where breastfeeding on demand typically continues for 18 to 36 months. Despite the merely suggestive nature of the original case reports and the subsequent lack of evidence linking breastfeeding to caries development, the American Academy of Pediatric Dentistry and some pediatric oral health experts nevertheless cautions against ad libitum breastfeeding after eruption of the first teeth. The objective of this proposal is to develop a comprehensive understanding of how human milk affect viability, colonization and caries development produced by the major pathogens responsible for ECC (Streptococcus mutans and Streptococcus sobrinus). In Aim 1, we will determine the ability of human milk to affect viability of mutans streptococci, to interfere with adherence to, biofilm formation on, and demineralization of enamel in vitro. In Aim 2, we will employ a weanling rat model to investigate colonization of mutans streptococci and caries development in vivo. We will determine whether human milk alone is cariogenic and whether human milk can protect against sucrose-induces colonization and caries development. We hypothesize that the anti-microbial activity of human milk will fail to support growth of mutans streptococci and causes inhibition of colonization and biofilm formation in vitro. We further hypothesize that human milk not only fails to cause caries but also that it protects against colonization, plaque formation and the progression of infection to tooth decay in vivo. This study has the potential to resolve the conflicting opinions held about breastfeeding and oral health in children by on the one hand oral health experts and on the other hand the American Academy of Pediatrics and The Institute of Medicine. The study will augment the very limited available information about human milk and caries development that shape current recommendations on breast-feeding children with deciduous teeth.